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| United States Patent |
6,264,736
|
|
Knopf
, et al.
|
July 24, 2001
|
Pressure-assisted molding and carbonation of cementitious materials
Abstract
A method is disclosed for rapidly carbonating large cement structures, by
forming and hardening cement in a mold under high carbon dioxide density,
such as supercritical or near-supercritical conditions. The method is more
reliable, efficient, and effective than are post-molding treatments with
high-pressure CO.sub.2. Cements molded in the presence of high-pressure
CO.sub.2 are significantly denser than otherwise comparable cements having
no CO.sub.2 treatment, and are also significantly denser than otherwise
comparable cements treated with CO.sub.2 after hardening. Bulk carbonation
of cementitious materials produces several beneficial effects, including
reducing permeability of the cement, increasing its compressive strength,
and reducing its pH. These effects are produced rapidly, and extend
throughout the bulk of the cement--they are not limited to a surface
layer, as are prior methods of post-hardening CO.sub.2 treatment. The
method may be used with any cement or concrete composition, including
those made with waste products such as fly ash or cement slag. Surface
carbonation is almost instantaneous, and bulk carbonation deep into a form
is rapid. By combining molding, curing, and carbonation into a single
step, carbon dioxide is better distributed throughout the entire specimen
or form, producing a uniform product.
| Inventors:
|
Knopf; F. Carl (Baton Rouge, LA);
Dooley; Kerry M. (Baton Rouge, LA)
|
| Assignee:
|
Board of Supervisors of Louisiana State University and Agricultural and (Baton Rouge, LA)
|
| Appl. No.:
|
170480 |
| Filed:
|
October 13, 1998 |
| Current U.S. Class: |
106/682; 106/738; 264/DIG.43 |
| Intern'l Class: |
C04B 022/06 |
| Field of Search: |
264/DIG. 43,333
106/682,738,742,752,817,820
|
References Cited
U.S. Patent Documents
| 3356779 | Dec., 1967 | Schulze.
| |
| 4069063 | Jan., 1978 | Ball | 106/713.
|
| 4093690 | Jun., 1978 | Murray | 264/82.
|
| 4117060 | Sep., 1978 | Murray | 264/82.
|
| 4350567 | Sep., 1982 | Moorehead et al. | 162/145.
|
| 4362679 | Dec., 1982 | Malinowski | 106/601.
|
| 4407676 | Oct., 1983 | Restrepo.
| |
| 4427610 | Jan., 1984 | Murray | 264/82.
|
| 5051217 | Sep., 1991 | Alpar et al. | 264/40.
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| 5307876 | May., 1994 | Cowan et al. | 106/790.
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| 5435843 | Jul., 1995 | Roy et al. | 106/705.
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| 5518540 | May., 1996 | Jones, Jr. | 106/638.
|
| 5650562 | Jul., 1997 | Jones, Jr. | 73/38.
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| 5690729 | Nov., 1997 | Jones, Jr. | 106/682.
|
| 5897704 | Apr., 1999 | Baglin | 106/696.
|
| 5918429 | Jul., 1999 | Hicks et al. | 52/181.
|
| Foreign Patent Documents |
| 644828 | Aug., 1984 | CH.
| |
| 4207235 | Sep., 1993 | DE.
| |
| 402018368 | Jan., 1990 | JP.
| |
| 406263562 | Sep., 1994 | JP.
| |
| 101077 | Jul., 1992 | RO.
| |
| WO9744294 | Nov., 1997 | WO.
| |
| WO9744293 | Nov., 1997 | WO.
| |
Other References
Bukowski, J. et al., "Reactivity and Strength Development of CO.sub.2
Activated Non-Hydraulic Calcium Silicates," Cem. Concr. Res., vol. 9, pp.
57-68 (1979).
Sauman, Z., "Effect of CO.sub.2 on Porous Concrete," Cem. Concr. Res., vol.
2, pp. 541-549 (1972).
Ota, Y. et al., "Preparation of Aragonite Whiskers," J. Am. Ceram. Soc.,
vol. 78, 1983-1984 (1995).
Bukowski, J. et al., Reactivity and Strength Development of CO.sub.2
Activated Non-Hydraulic Calcium Silicates, Cement and Concrete Research,
vol. 9, pp. 57-68 (1979).
Letter from Roger H. Jones, Jr. to Tom Peck II (Oct. 29, 1996).
Klemm, W. et al., "Accelerated Curing of Cementatious Systems by Carbon
Dioxide," Cem. Concrete Res., v. 2(5), pp. 567-576 (1972).
Knopf, F. et al., "Densification and pH Reduction in Cement Mixtures Using
Supercritical CO2," Abstract of paper presented at 1997 annual meeting of
the American Institute of Chemical Engineers, available on the Internet in
Jul. 1997 at
http://www1.che.ufl.edu/meeting/1997/annual/session/100/h/index.html.
Onan, D., "Effects of Supercritical Carbon Dioxide on Well Cements," Proc.
Annu. Southwest. Pet. Short Courts, 32nd, pp. 34-56 (1985).
Reardon, E. et al., "High Pressure Carbonation of Cementitious Grout,"
Cement and Concrete Research, vol. 19, pp. 385-399 (1989).
Bernard, A. et al., "Treatment of Cement Products with Carbon Dioxide,"
Comm. a l'Energie Atomique Fr., 5 pages (1969) Abstract.
Dewaele, P. et al., "Permeability and Porosity Changes Associated with
Cement Grout Carbonation," Cem. Concr. Res., v. 21(4), pp. 441-454 (1991)
Abstract.
Ohgishi, S. et al., "Relation Between the Accelerated Test Results and
Natural Progres for Carbonation in Concrete," Semento, Konkurito
Ronbunshu, v. 44, pp. 454-459 (1990) Abstract.
Simatupang, M., "The Carbon Dioxide Process to Enhance Cement Hydration in
Manufacturing of Cement-bonded Composites--Comparison with Common
Production Method," Inorg. -Bonded Wood Fiber Compos. Mater., v. 3, pp.
114-120 (1993) Abstract.
Sorochkin, M. et al., "Possible Use of Carbon Dioxide to Accelerate the
Hardening of Portland Cement Based Products," Zh. Prikl. Khim., v. 48(6),
pp.1211-1217 (1975) Abstract.
Dewaele, P.J. et al., "Permeability and Porosity Changes Associated with
Cement Grout Carbonation," Cement and Concrete Research, v. 21, pp.
441-454 (1991).
Simatupang et al., "The Carbon Dioxide Process to Enhance Cement Hydration
in Manufacturing of Cement-Bonded Composites--Comparison With Common
Production Method," Inorganic Bonded Wood Fiber Compos. Mater., v. 3, pp.
114-120 (1993).
|
Primary Examiner: Marcantoni; Paul
Attorney, Agent or Firm: Runnels; John H.
Parent Case Text
The benefit of the Oct. 15, 1997 filing date of provisional application
60/109,799 is claimed under 35 U.S.C. .sctn.119(e).
Claims
We claim:
1. A process for making a carbonated cement, comprising the steps of:
(a) placing an uncured cement comprising hydroxides of calcium into a
gas-tight compartment that contains the entire uncured cement;
(b) reacting the uncured cement with carbon dioxide that is introduced into
the gas-tight compartment at a pressure of at least about 400 psi, until
at least about 50% of the hydroxides of calcium have been converted to
calcium carbonate; wherein the ratio of the mass of introduced carbon
dioxide to the mass of the uncured cement prior to introduction of the
carbon dioxide is at least about 0.08; and
(c) curing the cement to form a hardened cement paste.
2. A process as recited in claim 1, wherein the carbon dioxide is a
supercritical fluid, or is a fluid whose carbon dioxide density exceeds
0.46 g/cm.sup.3.
3. A process as recited in claim 1, wherein the ratio of the mass of
introduced carbon dioxide to the mass of the uncured cement prior to
introduction of the carbon dioxide is at least about 0.12.
4. A cement produced by the process of claim 1.
5. A cement as recited in claim 4, wherein the porosity of said cement is
at least 50% lower than the porosity of a comparison cement that is
produced by an otherwise identical process, except that the comparison
cement is not reacted with carbon dioxide while curing, or is reacted only
with ambient carbon dioxide while curing.
6. A cement produced by the process of claim 2.
7. A process as recited in claim 1, wherein the uncured cement is admixed
with reinforcing polymeric fibers that are stable at the pH of the cured
cement.
8. A process as recited in claim 7, wherein said fibers comprise a
polyamide, a polyolefin, a polyamide blend, or a polyolefin blend.
9. A process as recited in claim 8, wherein said fibers comprise a nylon.
10. A process as recited in claim 8, wherein said fibers comprise
polypropylene.
Description
This invention pertains to carbonation of cementitious materials,
particularly to carbonation of cements using supercritical or high density
carbon dioxide.
Above a compound's "critical point," a critical pressure and temperature
characteristic of that compound, the familiar transition between gas and
liquid disappears, and the compound is said to be a "supercritical fluid."
Supercritical fluids (SCFs) have properties of both gasses and liquids, in
addition to unique supercritical properties. A supercritical fluid is
compressible like a gas, but typically has a density more like that of a
liquid. Supercritical fluids have been used, for example, as solvents and
as reaction media. The critical pressure and temperature for carbon
dioxide are 1071 psi and 31.3.degree. C. The viscosity and molecular
diffusivity of a supercritical fluid are typically intermediate between
the corresponding values for the liquid and the gas. Compounds below, but
near, the critical temperature and pressure are sometimes termed
"near-critical."
Hardened or cured cements have sometimes been reacted with high pressure or
supercritical CO.sub.2 to improve their properties. Supercritical and
near-critical CO.sub.2 increase the mobility of water that is already
present in the cement matrix, water bound as hydrates and adsorbed on pore
walls. A pore in the cement may initially contain supercritical or
near-critical CO.sub.2 at the pore entrance, a dispersed water phase
associated with the pore walls, and possibly free water at the CO.sub.2
/water interface. The high CO.sub.2 pressure increases the solubility of
CO.sub.2 in the dispersed aqueous phase. A concentration gradient of
CO.sub.2 is thus produced in the concrete pores. Carbon dioxide may then
react with various cement components, particularly hydroxides of calcium.
(As used in the specification and claims, the term "hydroxides of calcium"
includes not only Ca(OH).sub.2, but also other calcareous hydrated cement
components, e.g., calcium silicate hydrate.)
Densification Reactions
Carbonation reduces the permeability of cement, typically by 3 to 6 orders
of magnitude. This reduction in permeability has been attributed to
precipitation of carbonates in the micropores and macropores of the
cement. For example, in cement grout carbonation shifts a bimodal pore
distribution (pores around 2-10 nm in diameter and pores around 10-900 nm)
to a unimodal distribution (pores around 2-10 nm in diameter only).
Reduced permeability and smaller pore diameters slow rates of diffusion in
carbonated cements. For example, Cl.sup.- and I.sup.- diffusion
coefficients have been reported to be 2 to 3 orders of magnitude lower in
carbonated cement than in noncarbonated cement, as have carbon-14
migration rates. (Lower Cl.sup.- and I.sup.- diffusion rates indicate
greater resistance to salt intrusion. Salt intrusion is undesirable, as it
can lead to fracturing or cracking.) Curing cement grout with carbon
dioxide increases the strength and dimensional stability of a cement. The
pH of cement in fully carbonated zones is lowered from a basic .about.13
to a more neutral value of .about.8, allowing the reinforcement of the
cement with polymer fibers such as certain polyamides (e.g., nylons) that
are incompatible with normal cements.
Carbonation of cement is a complex process. All calcium-bearing phases are
susceptible to carbonation. For calcium hydroxide (portlandite) the
reaction is
Ca(OH).sub.2 +CO.sub.2.revreaction.CaCO.sub.3 +H.sub.2 O
The calcium carbonate may crystallize in one of several forms, including
calcite, aragonite and vaterite. Calcite is the most stable and common
form.
In this reaction, calcium hydroxide (Ca(OH).sub.2) is assumed first to
dissolve in water, after which it reacts with CO.sub.2. Following
reaction, the calcium carbonate (CaCO.sub.3) precipitates. Atmospheric
concentrations of CO.sub.2 (.about.0.04%), do not react appreciably with
completely dry concrete. Conversely, if the concrete pores are filled with
water, carbonation at low pressure essentially stops before bulk
carbonation of a thick cement form can occur, because the solubility and
diffusivity of CO.sub.2 in water are low under such conditions. However,
bulk carbonation of cement can occur at atmospheric pressure and ambient
temperatures after years of exposure to atmospheric carbon dioxide.
High pressure conditions have previously been used to carbonate the surface
layers of hardened cements. However, problems resulting from bulk
carbonation of hardened cements have been reported. For example, the
volume changes associated with conversion of calcium hydroxide to calcium
carbonate have been reported to cause microcracking and shrinkage, at
least under certain conditions.
Supercritical Fluids in Cementitious Materials
Supercritical and near-critical fluids confined in narrow pores have
properties that are often quite different from those of a bulk gas.
Because supercritical fluids are highly compressible, a surface or wall
potential can produce a strong, temperature-dependent preferential
adsorption, which might not occur at all at lower fluid densities. For
example, a water layer on the solid surfaces is believed to be necessary
to initiate carbonation reactions. Water is, in turn, a product of
carbonation. At lower pressures water can completely fill the pores and
thereby limit or even prevent carbonation; in such cases the sample must
be dried for carbonation to resume. However, saturation and
supersaturation of water in a CO.sub.2 -rich phase is possible at high
pressure, because phase separation in the concrete pores is slower than
the carbonation reaction. Also, at high pressures carbon dioxide may
adsorb onto the solid surfaces, along with water. The pore environment may
eventually consist of a fluid phase of water and dissolved CO.sub.2, with
mostly water but some CO.sub.2, adsorbed onto the walls of the concrete
pores. At high pressures solubility of CO.sub.2 in water increases.
E. Reardon et al., "High Pressure Carbonation of Cementitious Grout,"
Cement and Concrete Research, vol. 19, pp. 385-399 (1989) discloses
treating a solid, hardened, cementitious grout with carbon dioxide gas at
pressures up to 800 psi, and notes that this process can sometimes cause
physical damage to specimens, including fracturing due to dehydration and
shrinkage.
J. Bukowski et al., "Reactivity and Strength Development of CO.sub.2
Activated Non-Hydraulic Calcium Silicates, Cement and Concrete Research,
vol. 9, pp. 57-68 (1979) discloses treating non-hydraulic calcium
silicates with CO.sub.2 up to 815 psi, and notes that both the extent of
the carbonation reaction and the compressive strength of the carbonated
materials increased with treatment pressure.
U.S. Pat. No. 4,117,060 discloses a method for the manufacture of concrete,
in which a mixture of a cement, an aggregate, a polymer, and water were
compressed in a mold, and exposed to carbon dioxide gas in the mold prior
to compression, so that the carbon dioxide reacts with the other
ingredients to provide a hardened product.
U.S. Pat. No. 4,427,610 discloses a molding process for cementitious
materials, wherein the molded but uncured object is conveyed to a curing
chamber and exposed to ultracold CO.sub.2.
U.S. Pat. No. 5,518,540 discloses treating a cured cement with dense-phase
gaseous or supercritical carbon dioxide. The patent also mentions using
supercritical carbon dioxide as a solvent to infuse certain materials into
a hardened cement paste. See also U.S. Pat. No. 5,650,562.
U.S. Pat. No. 5,051,217 discloses a continuous stamping and pressing
process for curing and carbonating cementitious materials. CO.sub.2 was
admitted at low pressures, and could later be compressed to higher
pressures in one segment of the apparatus, a segment through which an
afterhardening cement mixture passed continuously. The apparatus was said
to be quasi-gas-tight. Only a portion of the uncured form was subjected to
high pressure at any given time. The ratio of the mass of CO.sub.2 to the
mass of the uncured cement was relatively low, apparently always under
0.002 (extrapolating from data given in the specification).
F. Knopf et al., "Densification and pH Reduction in Cement Mixtures Using
Supercritical CO.sub.2," Abstract of paper to be presented at 1997 annual
meeting of the American Institute of Chemical Engineers, available on the
Internet in July 1997 at
http://www1.che.ufl.edu/meeting/1997/annual/session/100/h/index.html
discloses some of the inventors' own work, work that is disclosed in
greater detail in the present specification.
We have discovered that a superior method to rapidly carbonate large cement
forms or structures is to shape and harden the cement in a mold under high
carbon dioxide pressure, at supercritical, near-supercritical, or high
CO.sub.2 density conditions. In other words, contrary to previous
teachings, supercritical, near-supercritical, or high density CO.sub.2 is
reacted with cement while the cement is still in an uncured state. The
novel carbonation method is more reliable, efficient, and effective than
are post-molding treatments with high-pressure CO.sub.2, or treatments
using low temperature, low pressure CO.sub.2. The novel method is more
effective and reliable than methods that admit relatively small amounts of
CO.sub.2 to a mold at relatively low pressure, and then compress the
uncured mixture. The novel method is more effective in penetrating voids
with CO.sub.2, and is therefore more efficient in converting hydroxides of
calcium to CaCO.sub.3. Cements molded in the presence of high-pressure
CO.sub.2 are significantly denser than otherwise comparable cements having
no CO.sub.2 treatment, and are also significantly denser than otherwise
comparable cements treated with CO.sub.2 after hardening.
The novel bulk carbonation of cementitious materials produces several
beneficial effects, including reducing permeability of the cement,
increasing its compressive strength, and reducing its pH. These effects
are produced rapidly, and extend throughout the bulk of the cement--they
are not limited to a surface layer, as are prior methods of post-hardening
CO.sub.2 treatment. The novel method may be used with any cement or
concrete composition, including those made with waste products such as fly
ash or cement slag. Surface carbonation is almost instantaneous, and bulk
carbonation is rapid even with forms several centimeters thick, tens of
centimeters thick, or thicker. By combining molding, curing, and
carbonation into a single step, carbon dioxide is better distributed
throughout the entire specimen or form, producing a uniform carbonated
cement product. In particular, it is believed that this is the first cured
cement in which all interior portions of the cement that are at least 1 mm
from the nearest surface of the cement comprise interlocking calcium
carbonate crystals that are at least 10 .mu.m in diameter.
Bulk carbonation of cement with supercritical CO.sub.2 in our laboratory
has produced a dense layer of interlocking calcium carbonate (calcite)
crystals in minutes. The crystals are an order of magnitude larger in
diameter (.about.10 .mu.m) than has been previously reported for calcite
crystals in the interior of cements. The novel process produces concretes
with improved durability and higher compressive strengths.
Uses for concretes based on the novel, bulk-carbonated cements are
numerous. The higher compressive strength allows the use of thinner blocks
and less material for a given strength requirement. For example, the
stronger concrete may be used to make lighter weight, fire-resistant
structural panels or roofing tiles. Cement roofing is rapidly gaining
acceptance. These roofs last essentially for the lifetime of the home,
have a Class A fire rating, and can be cast into any desired appearance.
Costs should be competitive with those for shorter-lived asphalt roofing
materials.
Low-cost reinforcing fibers may be used in bulk carbonated cements due to
the near-neutral pH of these materials. Many potential reinforcing fibers
are incompatible with the higher pH found in most cements, e.g. the pH
.about.13 of conventional Portland cements. For example, it has been
estimated that 3-4 billion pounds of carpet fiber per year are land-filled
in the United States. Recycled carpet polymers could instead be used to
reinforce these cement structures of near-neutral pH, transforming old
carpets from a waste product into a useful resource.
Carbonated cementitious materials can also be used for building artificial
reefs. Near-neutral pH's are necessary for the growth of most marine
organisms.
Carbonation and polymer reinforcement produce concretes with greater
resistance to chemical attack, a property that is useful, for example, in
the petroleum, mining, metallurgical, and chemical industries.
Bulk-carbonated cements have essentially no die-swell or warpage, an
advantage in the ceramics industry.
Preparation of Carbonated and Molded Samples
Comparison samples using previously cured cements were prepared in an
existing SCF continuous treatment system. Liquid CO.sub.2 was compressed
by a positive displacement diaphragm compressor (American Lewa model
ELM-1) to 1500 psi. The compressed CO.sub.2 was stored in surge tanks to
dampen pressure fluctuations. The pressure was controlled by a Tescom
regulator (model 44-1124) to within .+-.5 psi. Pressure was monitored by a
Heise digital pressure gauge (model 710A). The specimen (10 mm by 10 mm by
40 mm) was held in a tube immersed in a Plexiglas 25.degree. C. constant
temperature bath. The CO.sub.2 flow rate was .about.0.8 g/s, and the run
time was 1 hour.
A prototype device was constructed to evaluate the novel one-step method
for molding, curing, and supercritical (or near-critical or high density)
CO.sub.2 treatment. Specimens were treated in a simple cylindrical mold
operated by a piston, which was sealed on its outer surface by O-rings.
CO.sub.2 gas (at .about.700 psi) was introduced below the piston. The
pressure above the piston was rapidly increased using water as a driver
fluid. The increased pressure initiated the molding process. As the piston
moved rapidly toward the sample, the gas pressure above the sample rose to
equalize. But simultaneously the CO.sub.2 reacted with the cement, tending
to lower the pressure. A 2000 psi water pressure was applied to the
piston, and the samples were generally molded for .about.3 hours, although
shorter or longer times can be used. The molded specimens in the prototype
embodiment were cylindrical, 39 mm diameter by 13 mm height. The prototype
unit allowed various modes of CO.sub.2 addition to be studied, without the
complexities inherent in filling the mold with uncured cements under
pressure. However, the scope of the invention is not limited by the manner
used to fill the mold. The amount of CO.sub.2 added to the cement matrix
could be readily controlled by adjusting the initial height of the piston
above the cement.
Characterization of Chemical and Physical Properties of Cements
The porosities of conventionally cast samples (i.e., conventionally molded
without high pressure CO.sub.2) and samples produced by the novel process
were determined indirectly by measuring surface areas at a fixed initial
composition. Higher surface areas are often associated with void-filling
and therefore with decreased pore volumes, when small pores are created
from larger pores without significant pore closure. The amount of nitrogen
or other inert gas adsorbed (in determining surface area) includes
contributions from capillary condensation in small pores. However, as
voids are completely filled surface areas decrease significantly. A
discussion of physical adsorption mechanisms in porous materials can be
found in standard works on this subject, for example, D. M. Ruthven,
Principles of Adsorption and Adsorption Processes (1984).
Thus an increase in surface area upon carbonation indicates a small
reduction in voidage, while a decrease in surface area indicates almost
complete closure of voids in the specimen, accompanied by densification.
Surface areas were estimated using the one-point BET method at 30%
relative saturation, using a Micromeritics 2700 Pulse Chemisorption
apparatus. Water was first removed under vacuum at 1 torr for 24 h at
ambient temperature, then under flowing N.sub.2 /He for at least 2 h. The
surface areas of selected samples were checked by the full BET N.sub.2
adsorption method using an Omnitherm (model Omnisorp 360) adsorption
apparatus. The pore volume was determined in water by displacement
(Archimedes' principle). All specimens used in density and porosity
measurements were dried under vacuum at 1 torr at ambient temperature
prior to measurement.
A Scintag PAD-V automated X-ray Powder Diffractometer was used to identify
crystalline phases. Specimens were step-scanned from 3-60.degree.
2.theta., at a 0.02.degree. step size, 3 second/step. A Perkin-Elmer
thermogravimetric analyzer was used to quantify weight losses from water
evolution (from hydrates), hydroxide (e.g., Ca(OH).sub.2) to oxide (e.g.,
CaO) conversions, and carbonate (e.g., CaCO.sub.3) to oxide (e.g., CaO)
conversions. The carrier gas was helium at 1 atm. The temperature program
was 200-700.degree. C., 5.degree. C./min, hold at 700.degree. C.
Results, Post-Treated Samples
The "post-treatments" (i.e., carbonations of previously cast samples) used
near-critical CO.sub.2 (1500 psi and 25.degree. C.). The CO.sub.2 density
at these conditions was 0.83 g/cm.sup.3, well above the density at the
critical pressure and temperature (0.46 g/cm.sup.3). Table 1 summarizes
X-ray diffraction (XRD) results for five different concrete mixes. The
samples for the XRD measurements were taken from the surfaces of the
specimens. For each mix both a control sample (no carbonation) and a test
sample (carbonated) were measured. The reported weights of the additives
were normalized to the initial weight of concrete. For all samples, a
weight ratio of 0.603 water to 1.0 cement (ASTM Type III) was used in the
initial mix. The five mixes represent typical fast set concretes, some of
which included one or more of the following additives: glass fibers,
Kevlar fibers, calcite, lime, and a plasticizer.
TABLE 1
XRD Phase Characterization of Carbonated Specimens,
Continuous Flow Treatment
Ratio of XRD peak heights,
portlandite/calcite Additives
Control 1 2.9 none
Test 1 0.029
Control 2 3.9 0.021 lime
0.007 calcite
Test 2 <0.035
Control 3 4.1 0.021 calcite
0.022 WRDA 19
Test 3 0.08 plasticizer
Control 4 2.6 0.105 lime
0.021 calcite
Test 4 0.09
Control 5 3.6 0.105 lime
0.021 calcite
0.022 WRDA 19
plasticizer
Test 5 <0.035 0.007 E-glass fiber
0.010 Kevlar 49 fiber
The portlandite peak reported in Table 1 occurred at 18.1.degree. 2.theta.,
and the calcite peak at 29.5.degree.. The reported ratios of portlandite
to calcite are not strictly quantitative, because detailed calibrations of
peak height versus the weight of a given phase were not made, and also
because careful microtome sectioning procedures were not used.
Nevertheless, the five control samples showed a reasonably consistent
ratio range, 2.6-4.1.
As compared to the controls, the test samples showed a significant increase
in calcite (CaCO.sub.3) peak heights, and a corresponding decrease in
portlandite (Ca(OH).sub.2) peak heights. The relative ratio of P/C
(portlandite/calcite) for the control and test samples (i.e.,
(P/C).sub.control /(P/C).sub.test) ranged from a low of 29 for sample 4 to
a high of 111 for sample 2. Despite the semi-quantitative nature of these
initial XRD measurements, it is still clear that carbonation caused a 1-2
order of magnitude change in the ratio of portlandite to calcite in
samples taken from the surface. These experiments show that the presence
of typical cement additives did not hinder the carbonation process
substantially.
Scanning electron microscope (SEM) photomicrographs showed qualitatively
similar appearances for control and test samples at magnification
33.times.: individual, rounded sand grains coated with the cement. At
higher magnifications, 650.times. and 3700.times., significant differences
in the crystalline structures became apparent. Before carbonation, the
cement comprised primarily calcium silicate hydrate, calcium hydroxide,
and ettringite. The carbonated cement, by contrast, showed large calcium
carbonate crystals (average diameter 10 .mu.m), with partially developed
crystal faces. The average grain size was an order of magnitude greater
than that previously reported for carbonated cements. The calcium
carbonate crystals formed interlocking grains, suggesting that
permeability of the cement was thereby reduced. Also, adhesion between the
carbonated layer and the noncarbonated layer, as well as adhesion between
the carbonated layer and aggregate, both appeared to be good.
Derivative thermogravimetric analysis (TGA) of a Portland cement mortar
before and after carbonation was used to estimate content of calcium
carbonate and hydroxides of calcium. The complex chemical nature of a
typical cement precludes exact quantitation by TGA, so the TGA results are
considered to provide relative comparisons only. A large increase in
calcium carbonate content following carbonation was evident, as was a
proportional decrease in the content of hydroxides of calcium. The content
of ettringite and other stable hydrates appeared to be unaffected by the
carbonation.
The SEM micrographs suggested that surface carbonation was extensive.
Derivative thermogravimetry, on the other hand, indicated that about half
of the hydroxides of calcium did not undergo any change. This discrepancy
is explained by the fact that the SEM probed only the top few micrometers
of the surface, while the thermal analysis was representative of the top
several millimeters of the sample. Thus the deeper one probed into the
sample, the lower the degree of carbonation for the post-treated samples.
As shown below, the results were quite different for samples produced by
the novel supercritical molding treatment.
RESULTS, MOLDED SPECIMENS; AND COMPARISONS TO POST-TREATED SPECIMENS
The details of the treatments and initial compositions used in the molding
experiments are given in Tables 2 and 3. All initial cure times were 3
hours. All comparison samples were prepared in the molding device with
2000 psi water pressure on the driver side. Some comparison samples were
set with air only (i.e., with no more than ambient levels of CO.sub.2.)
Some comparison samples were set in air for three hours initially, and the
partially cured materials were then contacted with CO.sub.2 for an
additional two hours.
TABLE 2
Compositions and Treatments for Molded Portland Cement (PC) and Fly
Ash Samples
Mass
of PC Mass of 5 M NaOH
or Solution, as a
Fly Percentage Fiber Type and Mass, as
Sample Number Ash of Mass of a Percentage of Mass of
and Description (g) PC or Fly Ash PC or Fly Ash
3A- PC, set in air 50 32 polypropylene, 1.4
3B- PC, set with 50 32 polypropylene, 1.4
CO.sub.2
2A- fly ash, set in 25 40 none
air
2B- fly ash, set 25 40 none
with CO.sub.2
11- fly ash, 25 40 none
set with
water P = 2000
psi, then CO.sub.2
5- fly ash, set 25 45 polypropylene, 1.6
with CO.sub.2
15- fly ash, set in 25 40 polypropylene, 1.6
air, then CO.sub.2
16- fly ash, 25 44 nylon, 1.6
set with
CO.sub.2, foamed.sup.1
17- fly ash, 25 44 polypropylene, 1.6
set with
CO.sub.2, foamed.sup.1
.sup.1 foamed with aqueous solution comprising 73% 5 M NaOH and 27% aqueous
(30 wt %) H.sub.2 O.sub.2
TABLE 3
Compositions and Treatments for Molded Cement Slag Samples
Mass
of 5 N NaOH
Mass of Solution, Fiber Type and
Cement as a Percentage Mass, as a
Sample Number and Slag of Mass Percentage of
Description (g) of Slag Mass of Slag
4- set with CO.sub.2 25 44 polypropylene, 1.4
6- set in air, foamed.sup.1 25 44 0
8- set in air 25 40 polyproylene, 1.6
9- set with CO.sub.2 30 43 polypropylene, 4.3
10- set with CO.sub.2, 25 45 0
foamed.sup.2
12- set in air, CO.sub.2 post- 25 45 0
setting
.sup.1 foamed with aqueous solution comprising 55% 5 M NaOH and 45% aqueous
(30 wt %) H.sub.2 O.sub.2
.sup.2 foamed with aqueous solution comprising 76% 5 M NaOH and 24% aqueous
(30 wt %) H.sub.2 O.sub.2
For the fly ash and cement slag specimens, a 5 M NaOH solution was used to
reduce curing times, following the method of U.S. Pat. No. 5,435,843. In
some experiments, H.sub.2 O.sub.2 was used as a foaming agent to see
whether it would affect contact between the CO.sub.2 and the cements. The
Portland cement used was Type I. The fly ash was Class C. The cement slag
was standard pig-iron blast furnace slag.
After demolding, sectioned samples were tested for increases in carbonate
content by TGA. The reactions used to estimate Ca(OH).sub.2 and CaCO.sub.3
content were as follows:
Hydrates.fwdarw.Silicates, Carbonates (T<300.degree. C.)
MgCO.sub.3.fwdarw.MgO+CO.sub.2 [MW=44] (T.about.300-350.degree. C.)
Ca(OH).sub.2 [MW=74.1].fwdarw.CaO+H.sub.2 O [MW=18] (350.degree.
C.<T<450.degree. C.)
CaCO.sub.3 [MW=100.1].fwdarw.CaO+CO.sub.2 [MW=44] (T>600.degree. C.)
Hydroxylated Silicas, Aluminas.fwdarw.SiO.sub.2, Al.sub.2 O.sub.3 +H.sub.2
O (T<650.degree. C.)
Other Carbonates.fwdarw.Oxides+CO.sub.2 (T>500.degree. C.)
In most instances the MgCO.sub.3 peak could not be resolved from the
Ca(OH).sub.2 peak. Also, the final dehydrations of the surfaces of other
hydroxides such as SiO.sub.2, take place at temperatures that overlap
CaCO.sub.3 decomposition. The TGA results should therefore be viewed as
estimates of the amounts of Ca(OH).sub.2 and CaCO.sub.3 in these
materials. The TGA results are nevertheless useful in relative comparisons
of carbonated versus non-carbonated (but otherwise identical) materials.
Standard samples were used to calibrate appropriate temperature ranges for
the dehydration and decarbonation reactions in the TGA analysis. Each
standard was a homogeneous physical mixture, containing 2/3 mold specimen
3A (Portland cement, set in air), and 1/3 of the additive being tested.
These components were ground to a powder with a mortar and pestle. The
additives used in separate samples were as follows: CaCO.sub.3, which
produced a high-temperature reaction; Ca(OH).sub.2, which produced a range
of multiple dehydrations from .about.350-450.degree. C.; Al(OH).sub.3, for
which bulk dehydration occurred at low temperatures, in the hydrate-loss
region; and Na.sub.2 SiO.sub.3, which produced a peak at
.about.570-640.degree. C., an evolution of water from silicate surfaces
that can affect quantitation of the carbonate peak--however, the
relatively small size of this peak suggests that rough quantitation of
CaCO.sub.3 by TGA is still possible. Tables 4 and 5 give the TGA results
for the molded samples. In the Tables, the designations "M" and "T" refer
to samples that were removed from the middle of the specimen and the top
surface of the specimen, respectively.
TABLE 4
TGA Results, Fly Ash Samples
% Water Loss % Hydroxide as % Carbonate as
Sample from Hydrates Ca(OH).sub.2 CaCO.sub.3
2A - fly ash, set 2.8 9.6 6.5
in air
2B - fly ash, set 1.6 3.3 13.7
with CO.sub.2
11T 2.3 6.0 5.9
11M 2.7 5.2 6.0
5M 0.89 3.8 10.7
5T 0.74 4.0 11.0
15 - fly ash, set in 0.68 13.3 15.1
air, CO.sub.2 post-
setting
16 - fly ash, set 0.55 3.7 14.5
with CO.sub.2, foamed
TABLE 5
TGA Results, Cement and Cement Slag Samples
% Water Loss % Hydroxide as % Carbonate as
Sample from Hydrates Ca(OH).sub.2 CaCO.sub.3
3A, set in air 1.5 16.0 5.1
3B, set with CO.sub.2 1.3 13.4 7.0
4, slag, set with 0.86 18.5 7.7
CO.sub.2
6, slag, set in air, 0.45 14.4 1.1
foamed
8, slag, set in air 1.8 14.7 3.8
9 - slag, set 1.1 16.7 5.8
with CO.sub.2
10 - slag, set with 0.56 12.6 2.3
CO.sub.2, foamed
12 - slag, set 1.2 12.4 1.6
in air, CO.sub.2
post-setting
Note in Table 4 a general increase in measured CaCO.sub.3 content for all
the CO.sub.2 -molded samples as compared to the non-carbonated samples.
When CO.sub.2 was not used directly in the molding process, but was
instead applied as a post-cure treatment in the mold, the measured
carbonate content sometimes increased (sample 15), and sometimes did not
(sample 11). In Table 5 the carbonate content increased where CO.sub.2 was
used in the molding, except for one of the H.sub.2 O.sub.2 -foamed
samples. However, the carbonate content did not increase when CO.sub.2 was
used to treat an already-hardened cement slag (sample 12). These
experiments show that the high-pressure CO.sub.2 molding process is more
reliable and effective than is a post-molding treatment with high pressure
CO.sub.2.
The CO.sub.2 in-situ molded specimens were also denser than the air-molded
samples, as seen in Tables 6 and 7. Because carbonation filled pores and
cracks in the cement, the dry surface area should decrease upon
significant carbonation, as seen in Tables 6 and 7, even when polymer
fibers were present. The bulk density of the dry carbonated materials
increased, as carbonates are generally denser than hydroxides--with one
exception, sample 16, which was a H.sub.2 O.sub.2 -foamed sample (compared
to non-foamed standard 2A). Similar results were found for cement slags.
(See Table 7.) Note that the voidages for the carbonated samples decreased
significantly as compared to samples set in air (Tables 6 and 7). These
lower voidages demonstrate that the novel carbonated cementitious
materials possess excellent barrier properties, e.g. to ionic transport.
The decreased ionic permeabilities lend these cements to uses such as
housing and marine applications. In addition, reinforcing polymer fibers
blended with such cements would be less susceptible to degradation by
reaction with ions transported in water, especially saltwater or
wastewater.
TABLE 6
Porosity and Density Results, Fly Ash Samples
BET Voidage based on water
Surface Bulk displacement: (sample volume -
Area, density, water displaced)/sample
Sample m.sup.2 /g kg/m.sup.3 volume
2A - fly ash, set 8.5 1.82 0.18
in air
2B - fly ash, set 5.4 1.94 0.065
with CO.sub.2
5 - fly ash, set 8.4 1.95 0.049
with CO.sub.2
15 - fly ash, set 5.7 0.049
in air, then CO.sub.2
16 - fly ash, set 4.6 1.68 0.089
with CO.sub.2, foamed
17 - fly ash, set 7.0 1.80 0.015
with CO.sub.2, foamed
TABLE 7
Porosity and Density Results, Cement Slag Samples
BET Bulk Voidage (based on water
Surface density, displacement: (sample volume -
Sample Area, m.sup.2 /g kg/m.sup.3 water displaced)/sample volume
4 - set with 3.8 2.00 0.14
CO.sub.2
8 - set in air 5.8 1.69 0.29
9 - set with 0.4 1.93 0.14
CO.sub.2
10 - set with 4.7 1.93 0.15
CO.sub.2, foamed
This process can be conducted at any pressure above .about.400 psi,
preferably between .about.600 psi and .about.2000 psi. Although there is
no upper limit on pressure, as a practical matter it becomes increasingly
more difficult to handle fluids above a pressure .about.5000 psi. A
delivery pressure to the mold of .about.700-800 psi is particularly
convenient in many applications, because this is the pressure at which
carbon dioxide is delivered from a tank of liquid carbon dioxide at room
temperature (i.e., this is the vapor pressure of carbon dioxide at room
temperature). Subsequent molding would increase the pressure within the
mold. The temperature should be between .about.56.degree. C. (the triple
point of CO.sub.2) and 200.degree. C., preferably between 0.degree. and
50.degree. C. More specifically, the pressure/temperature combination
should be such as to produce a CO.sub.2 density near or exceeding the
critical density of CO.sub.2, 0.46 g/cm.sup.3. For example, at 25.degree.
C. the density of CO.sub.2 in a near-critical state of 1000 psi is 0.74
g/cm.sup.3. This density easily suffices to give uniformly carbonated
products.
As used in the specification and claims, unless context clearly indicates
otherwise, the term "carbon dioxide" refers to any liquid, gas, or
supercritical fluid containing a substantial amount of CO.sub.2, at least
20% by weight (as measured before reaction with, or dilution into, other
components). The term "cement" or "cementitious material" refers to any
calcareous material which, when mixed with appropriate amounts of water
(and, optionally, other curing additives), can be used as a binder for
aggregates formed from materials such as sand, gravel, crushed stone,
organic polymers, and other materials. A cement may include such aggregate
or polymeric materials as blended mixtures. Examples of cementitious
materials include Portland cements, fly ash, and cement slags such as
blast furnace slag.
The complete disclosures of all references cited in this specification are
hereby incorporated by reference. In the event of an otherwise
irreconcilable conflict, however, the present specification shall control.
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